1. Field of the Invention
This invention relates to circuit breakers provided with main contacts and, in parallel therewith, resistance contacts, and in particular relates to circuit breakers fitted with a parallel resistor having improved resistance contacts.
2. Description of the Prior Art
Use is made, for example, in switchgears of power circuit breakers, or systems in which, as shown in FIG. 1, there are provided main contacts 1 and, electrically in parallel therewith, resistance contacts 2, the resistance being inserted in the circuit when the main contacts 1 are closed or when they are opened. This is for various reasons, which include the need to restrict abnormal voltages which are produced during switching, or to raise the switching capability of the contacts by limiting the rate of rise and the peak value of the voltage which is generated between the contacts after circuit-breaking. By the use of such a system not only can the abnormal overvoltages be suppressed, but also the life of the main contacts 1 can be increased and the reliability of the device can be improved.
A resistor material which was previously used to meet this objective used Al.sub.2 O.sub.3 replaced by SiO.sub.2 or the like. However, with the trend to larger capacity switchgear units, the parallel resistor also becomes larger, which militates against the trend to improved compactness of the device.
The resistance of the resistor is determined by the circuit to which it is applied and the overall application, but to suppress overvoltages generated when the main contacts are closed, it is necessary to make the resistance comparatively low (on the order of several hundred ohms). The heat which is generated by the resistor is proportional to the square of the applied voltage and inversely proportional to the resistance. Thus, if the voltage is high, an enormous amount of heat is generated by the resistor when the current is passed. Since this heat is generated instantaneously, it cannot be expected that it will be radiated from the resistor, and so it accumulates in the resistor material. In general the permissable rise in temperature of the switchgear has a limit, and if the temperature rises beyond this, the material swells up or becomes weakened, causing a deterioration in its electrical and mechanical properties and a decline in insulation strength. To control the rise in temperature of the switchgear, therefore, conventional resistors were of large volume, resulting in a large device being necessary.